Binders or binder systems for foundry cores and molds are well-known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern, and then cured with the use of catalysts and/or heat to a solid, cured state.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at low temperatures. These processes are preferred over high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as no-bake processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern. Curing of the binder is achieved without heating.
One such no bake process employs an aqueous alkaline solution of a phenolic resole resin as the binder. In this process, the foundry sand is usually mixed with an ester hardener before the solution of resole resin is added to the mixture. The process is described in detail in U.S. Pat. No. 4,474,904 (Re 32,812) which is incorporated herein by reference in its entirety.
The ester cured process is superior to some of the earlier processes from an environmental standpoint. However, the initial tensile strengths of the cores made by this process tend to be somewhat lower than those prepared by other no bake processes.
In Japanese Patent Application 62-282743 the absolute amount of binder addition was increased, resulting in higher tensile strengths of ester cured resins. This was achieved by using a solution of a phenolformaldehyde resin in the organic ester as the hardening agent. Examples of both novolak and resole resins in the ester solution were given. However, this approach has its limitations. The novolak resins used dissolve only in gamma-butyrolactone and are not soluble in other ester hardeners which are useful in many applications in alkaline phenolic resin binders. The resoles disclosed do not form stable solutions in a number of the ester hardeners.
We have now discovered that solutions of benzylic ether phenolic resole resins in ester solvents are excellent hardeners for use in the ester cured process. Higher initial tensile strengths of cores made using this binder system are achieved without increasing the absolute amount of binder addition. Furthermore, the benzylic ether phenolic resole resins form stable solutions in triacetin (glycerol triacetate), ethylene glycol diacetate and other ester hardeners in which ordinary resole resins are not stable.